Low temperature techniques have found extensive uses in industry, in laboratories, and in the medical field.
The freezing of tissue samples prior to the preparation of biological samples for electronmicroscopy is an example of the use of a low temperature technique. The sample is normally plunged into boiling liquid nitrogen or into a cryogen such as one of the Freons that has been cooled down to a temperature below its boiling point.
It is well known that repeatability is an important feature in the freezing of biological tissue. If one sample is to be comparable with another then the factors which influenced the way in which the tissue froze must be as identical as possible in each case. Ice crystal formation in the tissue sample as it is being frozen is a cause of non-repeatability. A rapid rate of cooling is essential because slow cooling has the effect of promoting the growth of large ice crystals which lower the usefulness of the tissue sample due to deformation of its cell structure and redistribution of chemical components in the tissue. Fast cooling rates produce more ice crystals of smaller size with reduced tissue deformation.
The present invention seeks to provide cryogenic apparatus and a cryogenic method which enable experimental repeatability to be achieved where this is desired, and which also enable rapid freezing, and hence the elimination of large ice crystals, to be obtained.
Another use of cryogenic techniques involves introducing into, for example, boiling liquid nitrogen a material which has two or more components that have been homogeneously dispersed in the material by, for example, mechanical mixing. The components are dispersed in a liquid carrier medium, usually water, and the purpose of dripping them into boiling liquid nitrogen is to freeze the water as rapidly as possible. The frozen material is then subjected to sublimation i.e. is freeze dried to remove the water. The purpose of adopting this technique is to reduce re-distribution of the components of the material with respect to one another as a result of the growth of large ice crystals, which has the effect of destroying the homogenity which has been achieved by mixing. The more rapid the cooling the smaller and more numerous the ice crystals that are formed. Slow cooling results in the formation of a smaller number of large ice crystals, the larger ice crystals displacing the components of the mixture and resulting in the mixture being non-homogeneous. The purpose of the freeze drying step is to avoid the carrier medium returning to its liquid state during the drying process as this would result again in a rearrangement of the components.
Such techniques find application in the production inter alia of ceramics such as Beta Alumina ceramic, potassium superoxide, and catalysts.